This invention relates generally to the art of filtering, and is concerned in particular with the filtering of pressurized material in flowable form.
There are numerous commercial operations dealing with pressurized materials in flowable form where filter elements are employed to remove contaminants, impurities, etc. When the filter elements become clogged, they must be replaced, cleaned or renewed, and with certain operations this can present serious problems. An example of one such operation is the filtering of molten plastic being fed from an extruder, melt pump or the like to a die producing any one of a wide range of products, such as for example blow-molded bottles, thin-film plastic bags, etc. In operations of this type, the molten plastic is passed at elevated temperatures in excess of 200.degree.-300.degree. F. and high pressures typically on the order of 1000 to 5000 p.s.i. through a filter apparatus located between the extruder or melt pump and the molding machine. In addition to effectively removing contaminants from the melt stream, the filter apparatus must also operate safely and continuously over extended periods of time, with a minimum substantially constant pressure drop across the filter element. By minimizing the pressure drop across the filter element, the volume of plastic flow to the molding machine is maximized, thereby permitting higher production rates. Maintaining the pressure drop substantially constant simplifies the task of controlling the operation of equipment on both the upstream and downstream sides of the filter.
The prior art filter devices which have heretofore been developed have failed to meet the criteria of successful operation outlined above. More particularly, in earlier devices of the type described in U.S. Pat. No. 642,814, separate filter screen elements are located in two laterally spaced positions on a shuttle plate which can be reciprocated in opposite directions transverse to the flow axis of the material being filtered. Such devices are characterized by numerous drawbacks, including pronounced fluctuations in feed pressures to downstream equipment due to infrequent changes of clogged filter screen elements, difficulties in cleaning and/or replacing clogged filter screen elements, and dangerous leakage of hot high pressure molten material when the shuttle plate is shifted.
The filter devices described in U.S. Pat. Nos. 3,471,017 and 3,855,126 represent efforts to provide improved alternatives to the earlier devices employing shuttle plates. However, these later devices have also fallen short of providing a satisfactory solution due to their reliance on the solidification of plugs of molten material to provide seals for elongated filter screens being moved across the flow path of the material being filtered. The cool/heat cycles required to alternately set and melt such seals are difficult to control, particularly at the frequencies required to move the filter screens at sufficiently rapid intervals to avoid undesired pressure drops caused by the filter screens becoming clogged. If the cool/heat cycles are not carefully controlled, the sealing integrity provided by the seals is lost. When this occurs, even momentarily, operating personnel are exposed to a dangerous escaping stream of high temperature high pressure molten plastic, and the operating cycle of both upstream and downstream equipment is disrupted with attendant production losses.
The above problems were eventually solved by the filter described and claimed in copending application Ser. No. 537,780. With this improved filter, pressurized flowable material is conducted to and from a filter zone respectively by inlet and discharge pressure conduits. A filter element including an elongated perforate web is driven in one direction, either continuously or intermittently, into and out of the filter zone through respective entry and exit sections of a guide channel. The perforate web extends transversely across the fluid flow path, preferably while engaged upon a downstream perforate support member. A plurality of seal members extend across the perforate web transversely to and at fixed spaced intervals along the longitudinal axis thereof. The seal members are suitably dimensioned to cooperate in internal sealing engagement with imperforate walls of the guide channel sections.
By employing an elongated perforate web movable either continuously or intermittently across the filter zone, a readily replaceable fresh filter medium is available, with the result that the pressure drop across the filter apparatus is kept at a relatively constant minimum level. This maximizes the flow of the material being filtered, and also facilitates control of both upstream and downstream equipment. In addition, the pressurized flow of material is effectively contained by the internal sealing engagement of the seal members with the imperforate walls of the guide channel, the entry and exit sections of which have lengths with relation to the spacing between the seal members which insure that at least one seal member is present in each channel section at all times during the filtering operation. Reliance on temperature-sensitive plugs of flowable material, and the problems associated therewith, is thus avoided.
The present invention is directed to the filter element adapted for use with the filter apparatus described and claimed in the aforesaid copending application.
The filter element includes an elongated perforate web adapted to move longitudinally across the flow axis of the material being filtered and to accommodate filtered flow therethrough of the material. A plurality of seal members are fixed to the perforate web transversely to and at spaced intervals along the longitudinal axis thereof. The perforate web extends continuously and uninterruptedly across the seal members to thereby provide the sole and exclusive means of interconnection therebetween. The seal members preferably have a thickness at least as great or greater than the thickness of the perforate web.
The perforate web may comprise a single perforate strip, or alternatively, it may include a plurality of superimposed perforate strips held together at spaced intervals by the seal members. The perforate strips may comprise metal wire mesh, or other suitable filter media, including sintered metal, perforated metal, woven synthetic or natural filter materials, foamed materials, microporous plastics, etc.
At least one of the perforate strips making up the perforate web has high tensile strength characteristics in order to interconnect the seal members in a manner which enables them to withstand high operating pressures and also to enable the filter element to be driven longitudinally through the guide channels and across the filter zone. Such high strength tensile characteristics may be obtained by employing laterally spaced tension members, for example high strength metal wires, which extend longitudinally and continuously over the entire length of the perforate web, and which are held in laterally spaced relationship by woven fill wires and/or by the seal members. Alternatively, the perforate web may include a strip of perforated metal, plastic tape or any other suitable perforated material having the desired tensile strength characteristics.
In comparison to the perforate web, the seal members are relatively rigid and may be formed from any suitable material, or combination of materials including poly amide imide, metal, rubber, leather or fabrics. The seal members are preferably wider than the perforate web, with the ends of the seal members protruding laterally from the edges of the perforate web to locate, guide and resist twisting of the perforate web. The seal members may be fixed to the perforate web by any one of a number of known techniques, including thermal bonding, brazing, solvent welding, adhesion, sonic welding, molding in place, etc. The seal members may be either imperforate or perforate. Optionally, the seal members may be provided with lips which are adapted to be urged by the pressurized material outwardly away from the perforate web and into sealing engagement with surrounding guide surfaces. Preferably, the perforate web is located by the seal members in a plane at or close to the downstream sides of the seal members.
The filter element of the present invention is ideally suited for, although not limited to, filtering operations at elevated temperatures in excess of 200.degree.-300.degree. F, and at pressures typically on the order of 1000 to 5000 p.s.i. and higher.